StarCAT is a catalog of high resolution
ultraviolet spectra of objects classified
as "stars," recorded by Space Telescope
Imaging Spectrograph (STIS) during its initial seven years of
operations (1997-2004). StarCAT
is based on 3184 echelle mode observations of 545 distinct targets, with a
total exposure duration of 5.2 Ms.
For many of the objects, broad ultraviolet coverage has been achieved
by splicing together echellegrams taken in two
or more FUV (1150-1700 Å) and/or NUV (1600-3200 Å) settings.
In cases of multiple pointings on
conspicuously variable sources, spectra were separated into
independent epochs. In cases of nonvariable, or lightly variable, objects,
different epochs were combined to enhance signal-to-noise (S/N).

Here is the StarCAT Portal. Below is a brief
summary of the project. A
full description can be found in
StarCAT Summary. The author strongly
encourages consulting the latter
before attempting to use the StarCAT spectra for analysis purposes.

Brief Description

The StarCAT sample was assembled by two crosscutting searches through
the HSTonline catalog hosted at
MAST. The initial search focussed
on objects identified by the Guest Observer as "STAR-∗" for the first
broad category keyword. The second search focussed
on the second choices of GOs when the first choice was "STAR-∗."
(Sometimes, a GO would use these less preferred secondary
descriptors as the leading broad category keyword.)

A naming convention was adopted
to unify the somewhat diverse target IDs assigned by the GOs, taking
in order of priority: HD number, variable star name (including
supernovae: SN), WD number
(from McCook et al. 2006), and
assigning "STARHHMM±DDMM" to all others (equinox/epoch 2000). In
one case, "NOIDHHMM±DDMM" was tagged to an observation
lacking a SIMBAD object within 20″.

Post-processing began at the level of the calstis pipeline "x1d" file;
somewhat a misnomer because it actually is a 2D tabulation
of wavelengths, flux densities, photometric errors, and
data quality flags for the up to several dozen orders of the particular grating
setting (e.g., E140M-1425, where the first part is the mode and the second is
central wavelength in Å).
The x1d file contained at least one —
and sometimes several — subexposures, which were treated
as separate observations. The initial processing included a
post facto correction to compensate
for subtle wavelength distortions identified in a recent study of the
STIS dispersion relations
(the "Deep Lamp Project"); a recalculation of the
photometric error (replacing N&frac12 with [N+1]½); and more aggressive
edge trimming of E230 modes to avoid unflagged "dropouts" that sometimes
occurred at the beginning of the low orders.
The x1d orders
then were merged, averaging the overlap regions weighted by
the individual sensitivity functions s λ, but accounting for
bad pixels and wavelength gaps.

Next, a series of different layers
of coaddition and splicing were applied to the sets of order-merged 1D spectra
of each object.

STAGE ZERO — the subexposures, if any, of an observation
were combined. The individual spectra were aligned in velocity by
cross-correlation against the first observation. The initial exposure should have the
most reliable wavelength scale because it normally would have been taken
closest to an acquisition and peak-up (the zero point is dependent
on the accuracy of the target centering, which is affected over time
by jitter and/or drifts). The subexposures then
were interpolated onto the wavelength scale of the initial spectrum
and coadded, weighting by exposure time, taking into account bad pixels and
gaps. The resulting files are called
"o-type" and have the same rootname as the original STIS observation, e.g., o61s01010.

STAGE ONE — the independent exposures of like mode and aperture,
all taken in a well defined group within a single "visit," were
combined. As in Stage Zero, the spectra were aligned by cross-correlation
against the first of the sequence, interpolated onto that scale, then
coadded, again weighting by exposure time. The resulting files are
called "E-type," e.g., "E140M-1425_020X020_51613." The initial part is the particular mode/tilt
(here, "E140M-1425" for the medium-res FUV echelle mode with central wavelength
1425 Å), followed by an aperture code (here, "020X020"
for the 0.20″×0.20″ photometric slit),
and ending with the start time of the first exposure expressed in integer days
as a Modified Julian Date
(MJD: J.D. - 2,400,000).

STAGE TWO — like-mode exposures of an object taken in different visits
and/or using different apertures were combined. Now, the cross-correlation
alignments were done relative to the highest S/N exposure of the
group (perhaps a Stage 0 or 1 coadd), and the average shift was subtracted.
Also, the coaddition weighted the constituent spectra by a factor
closely related to the total net counts at each wavelength, to compensate
for the different transmission factors of, say, a narrow versus broad
slit. The multi-epoch coadditions were undertaken only if the object
displayed minimal variability during its timeline. The resulting files,
like Stage 1, are E-type, but with extended aperture codes and/or
MJDs to reflect the diversity of the constituent
exposures, e.g., "E230M-2707_020X020_020X006_51163-51202" for one
extreme example.

STAGE THREE — the available wavelength segments
of an object were spliced together, perhaps grouped by epoch if the object displayed
noticeable variability. Again, wavelengths of adjacent segments were aligned
by cross-correlation. Also, relative flux ratios were measured in the overlap zones to ensure
coherent spectral energy distributions for objects with broadly overlapping
coverage. The philosophy was to retain at each wavelength the
highest resolution fluxes available, and to minimize coadding (in
overlap zones) spectra of mixed resolution. The resulting file is called
"U-type," e.g., "UVSUM_1M_52755."
The "UVSUM" part signals that a multi-wavelength splice was involved;
the middle numeral points to a particular grouping of spectra that were
spliced; the adjacent letter tells whether the spectra all were
medium resolution ("M"), all were high resolution ("H"), or mixed
("X"); and the trailing date (or dates if a range) indicates the
extreme starting MJDs of the spliced group.

Contents of the Catalog

The top-level object list (StarCAT Portal), lists
brief stellar characteristics abstracted
from SIMBAD,
and links to several layers of data tables. It looks like this:

Notes.
Stellar data from
SIMBAD. Coordinates
(α, δ) are equinox/epoch 2000,
in degrees, of the HST pointing; V and B-V are in magnitudes; and
parallax (π) is in arcseconds. The uncertainty in π typically is ±0.001″.
Tildes indicate
missing, or uncertain, values.

The first layer down (linked through object name [Column 1])
represents the final spectrum (or spectra) for each object, which might be only a single
o-type, say from a GO "SNAPSHOT" program; or a single E-type if only one
mode/setting had been utilized, but, say, with several independent
o-type exposures; or a full-blown U-type covering all or part of the
1150-3200 Å range, perhaps as a function of
epoch for variable targets.

This "final spectra" layer then links down to the constituent spectra
in the splice, if
any; and each of these in turn links down to whatever grouping, if any,
of exposures constituted it. Thus, the lowest layers of a tree always
are the o-type exposures; the next one or two layers up are the E-types;
and finally the top layer holds the U-types (at best; an o-type at
worst; and E-types in intermediate cases).

There are several kinds of processed data. First, the
page will display a JPEG preview of the spectrum (or spectra, if more than
one at the top level). Also, at the top level, a graphical timeline of all the observations
is provided, color-coded by mode. Second,
FITS
files of the fundamental data are linked and can be downloaded.
Finally, for the top-level datasets, an "ETC-ready" ASCII file is
linked. It is a highly processed, compacted version of the final spectrum,
specifically intended to be used with an
HST Exposure Time Calculator
(any that adheres to the HST
ETC format standard). A
description of the compactification procedure
and numerous warnings concerning the use of these ETC files can be found in
ETC Summary.

Data Formats.
A word
concerning the FITS file formats.
The o-type files have basic
header information in the zeroth extension (EXTEN=0) concerning the target, exposure properties, and splice
points from the merging process;
and one or more trailing data extensions. If
the observation consisted of a single exposure, there would be only one data
extension — EXTEN=1 — and the spectral parameters would be found there.
The quantities stored are
WAVE — wavelength (Å); FLUX — flux density
(ergs/cm²/s/Å); ERROR — photomeric error (same
units as flux density); and DQ — data quality (0 for no issues; higher values to flag various
conditions such as bad pixels, camera blemishes, gaps, and so forth).
On the other hand, if there were two or more subexposures,
EXTEN=1 would hold the Stage Zero coadded spectrum, while the trailing
extensions would contain the parameters of the individual subexposures: subexp#1 in EXTEN=2,
subexp#2 in EXTEN=3, and so forth. The EXTEN=0 header now also would list
the cross-correlation template (a specific spectral feature, say an ISM absorption line, at wavelength λ,
and the correlation window Δλ) and the derived velocity shifts.

The E-type (coadded) and U-type (spliced) data files are analogous, and consist of two extensions.
EXTEN=0 again summarizes basic information concerning the target
and exposure properties, including
digests of cross-correlation templates, splice points, and flux scale factors (the
latter two applying to the U-types only). EXTEN=1 contains the
spectral parameters for the coadded or spliced spectrum.
In all cases, including o-types,
the most refined dataset always
is in EXTEN=1.

Final word.
The author would like to express his appreciation
to all of the STIS investigators who have indirectly, and unwittingly,
contributed to StarCAT. It was fascinating to view the wide
diversity of UV spectra available in the MAST archive for the stars, and the
often dramatic differences between objects. The fact that such
differences exist, and are widespread, is testament to the enormous value of UV
spectroscopy to stellar astronomy.

At the same time, the author cautions users of StarCAT that — as
with any large scale project like this — there are inevitable errors and
omissions that have escaped the albeit numerous checks that the author has
conducted. The author would appreciate any feedback along these lines,
or any other comments for that matter, so that corrections can be made
and/or enhancements undertaken to improve the catalog for future users.

Acknowledgments.
Based on observations made
with the NASA/ESA Hubble Space Telescope, obtained from the
Data Archive at the Space
Telescope Science Institute, which is operated by the Association of Universities for
Research in Astronomy, Inc., under NASA contract
NAS 5-26555. Support for StarCAT was provided by grant
HST-AR-10638.01-A from STScI, and grant NAG5-13058 from NASA.
The project has made use of public
databases hosted by SIMBAD and VizieR, both maintained by
CDS, Strasbourg, France.